1. Field of the Invention
The present invention relates to nuclear magnetic resonance devices, and in particular to a method for operating such a device.
2. Description of the Prior Art
Nuclear magnetic resonance devices are known in the art as exemplified by the apparatus described in U.S. Pat. No. 4,520,315. Such devices typically include coils for generating a fundamental magnetic field and coils for generating gradient magnetic fields in which an examination subject is disposed, a high-frequency means for generating a sequence of high-frequency pulses and for acquiring nuclear magnetic resonance signals from the examination subject.
Various operating modes for such devices are also known, such operation modes being characterized by differing chronological sequences of the high-frequency pulses and the gradient fields. The selection of a particular operating mode is essentially based upon the type of image information desired. Examples of such different operating modes are the spin-echo method (mixed-emphasis imaging with respect to T.sub.1 and T.sub.2), the inversion recovery method (T.sub.1 -emphasized imaging), the multi-slice method, and the multi-echo method. In general, a common goal is to obtain as much information as possible within short measuring times and with a high signal-to-noise ratio.
Imaging methods operating with steady state free precession (SFP) have become known as multiple sensitive point (MSP) methods as described in the scientific note "NMR Images By The Multiple Sensitive Point Method: Application To Larger Biological Systems," Andrew, et al, Phys. Med. Biol. 22, No. 5 (1977), pages 971-974. These methods, however, have not proven practical because they require modulated gradients thereby resulting in an imprecise slice selection. Moreover, the measuring time cannot be shortened because a slice selection is only possible at all by mean value formation.